The present invention relates to an image heating apparatus usable with a fixing device of an image forming apparatus such as a copying machine, printer or the like of an electrophotographic type electrostatic recording type. Referring first to FIG. 12, there is shown an example of an image heating device.
As shown in FIG. 12, a primary side heat generation pattern 101 (heat generating element) is provided on one of the surfaces of a ceramic plate 110C which is a substrate mainly comprising a ceramic material, and a chip thermister 104 (temperature detection member) is provided on the other surface of the ceramic plate 110C, so that primary side heat generation pattern 101 is supplied with electric power from a voltage source through a primary side wire 108, primary side electrode 102 and primary side connector 103, by which the heat is generated. A chip thermister 104 is connected to a micro computer or the like which is an external control means through a secondary side pattern 106, secondary side connector 107 and a secondary side wire 109.
However, in such an image heating apparatus, the electric energy consumption of the heat generation pattern 101 is normally large, and therefore, the commercial voltage source is used energizing it. The heat generation pattern 101 and the chip thermister 104 are primary and secondary side circuits. Then, a sufficient insulation distance is required between the heat generation pattern 101 and the chip thermister 104 for the purpose of protecting the user.
In such a heating apparatus, the primary side and the secondary example circuits are mixed, and therefore, as shown in FIG. 12, the primary side electrode 102 and the second side electrode 105 have to be disposed at one end and the other end, respectively, of the ceramic plate 110C. This reduces the latitude in design and liquid ejection leads to difficulty in downsizing.
If the primary side electrode 102 and the secondary side electrode 105 are both disposed at one side, it is required to use a larger ceramic plate 110C to assure the distance between the primary side electrode 102 and the secondary side electrode 105, and in addition, coating has to be made thicker to assure the withstand voltage between the primary side electric wire 108 and the secondary side electric wire 109, with the result of cost increase.
It is not possible to reduce the width of the ceramic plate 110 because the distance speculated in the relevant safety relation between the primary side electrode 102 and the secondary side electrode 105, and therefore, downsizing is difficult, and the ceramic material cannot be saved, with result of cost increase.
On the other hand, in the case of heat fixing device of a film heating type using as a heat source a thick film heat generating resistor formed on a ceramic plate, the temperature rise of the heater is quick, and therefore, quick start operation can be accomplished, and electric power consumption can be saved.
However, the small heater thermal capacity leads to the difficulty in control. Generally, a constant temperature control is effected in a heat fixing device in the image recording device, and an abrupt temperature change during image fixing operation is not preferable. In view of this, when a heat generating element is used as a heat source for the heat fixing device, the use is made with a heat generating element which having a rated electric power which is larger than the desired electric power value, in which the constant temperature control is accomplished using a phase control or a wave number control. Therefore, when the control circuit for the heat generating element fails such that electric energy is always supplied, the temperature of the heat generating element rises abruptly. If nothing is done against this, the fixing device smokes or fires sooner or later. In consideration of this, the heat fixing device is provided with a thermal protector such as a temperature fuse or the like.
However, the thermal capacity of the thermal protector such as a temperature fuse is generally large as compared with that of the heat generating element and/or the ceramic plate, and therefore, the response is slow. For this reason, the heater (the ceramic plate including the heat generating resistor) may be broken due to the heat stress prior to the operation of the thermal protector.
If this occurs, electric discharge occurs between film portions of the thick film heat generating resistor where it is broken. Since the temperature of the elements around the fixing device is high, the elements relatively easily smokes or fires. Therefore, even if the ceramic plate of the heater is broken due to the heat stress upon abnormal operation, the electric energy supply to the thick film heat generating resistor is stopped with certainty by means of circuit means, so that firing and smoking are prevented. Ordinarily, the circuit means for preventing the firing and smoking is disclosed that secondary side, that is, the temperature detection member side.
Accordingly, it is a principal object of the present invention to provide an image heating device having a high safety with respect to the heat generating element. It is another object of the present invention to provide an image heating device in which a sufficient distance can be assured between the heat generating element and the safety circuit for stopping electric energization upon abnormality. It is a further object of the present invention to provide an image heating device in which the temperature detecting member and the abnormality detecting means are connected through a photo-coupler.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.